1. Field of the Invention
The present invention relates to a power supply for a load control device, and more particularly, to a cat-ear power supply that is able to charge an energy storage capacitor from an alternating-current (AC) power source at the beginning and the end of each half-cycle of the AC power source.
2. Description of the Related Art
A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus to control the intensity of the lighting load. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.
Smart wall-mounted dimmers may include a user interface typically having a plurality of buttons for receiving inputs from a user and a plurality of status indicators for providing feedback to the user. These smart dimmers typically include a microprocessor or other processing device for allowing an advanced set of control features and feedback options to the end user. An example of a smart dimmer is disclosed in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety.
In order to provide a direct-current (DC) voltage VCC to power the microprocessor and other low-voltage circuitry, the smart dimmers typically include cat-ear power supplies. A cat-ear power supply draws current only near the beginning and the end of a half-cycle of the AC source voltage and derives its name from the shape of the current waveform that it draws from the AC voltage source. Because the smart dimmer only has two terminals, the power supply must draw current through the connected lighting load. In order for the power supply to be able to draw sufficient current, the semiconductor switch must be non-conductive so that a sufficient voltage is available across the power supply. Thus, the semiconductor cannot be turned on for the entire length of a half-cycle, even when the maximum voltage across the lighting load is desired.
FIG. 1 is a simplified schematic diagram of a prior art cat-ear power supply 10. The power supply 10 comprises an energy storage capacitor C12 across which a DC supply voltage VC is produced. The power supply 10 also comprises a full-wave bridge rectifier BR14 that is coupled to an AC power source and produces a rectified voltage V+. The capacitor C12 is operable to charge through the rectifier BR14, a switching circuit 20, and a diode D16 to generate the supply voltage VC.
The switching circuit 20 comprises a semiconductor switch (e.g., a FET Q22), having drain and source terminals coupled in series electrical connection between the rectifier BR14 and the capacitor C12. A gate of the FET Q22 is coupled to a drive source circuit 30 through resistors R24, R26, R28. The drive source circuit 30 generates a drive voltage, which is used to control the FET Q22 into the conductive state. To produce the drive voltage, a capacitor C32 charges from the rectified voltage V+ through a resistor R34 and a diode D36. During each half-cycle of the AC power source, the capacitor C32 begins to charge when the magnitude of the AC voltage exceeds the voltage across the series combination of the capacitor C32 and the capacitor C12. The voltage across the capacitor C32 is limited to approximately a breakover voltage of a zener diode Z38 (e.g., approximately 40 V). When the voltage at the gate of the FET Q22 exceeds a predetermined gate voltage (e.g., approximately 15 volts), the FET is rendered conductive allowing the capacitor C12 to charge. A zener diode Z29 of the switching circuit 20 prevents the voltage at the gate of the FET Q22 from exceeding a predetermined safe operating voltage (e.g., 25 V) to protect the FET.
An overcurrent protection circuit 40 is coupled in series between the switching circuit 20 and the capacitor C12. A sense voltage generated across a sense resistor R42 is representative of the magnitude of the current through the capacitor C12. If the current through the sense resistor R42 exceeds a predetermined current limit, an NPN bipolar junction transistor Q44 is rendered conductive. Accordingly, the gate of the FET Q22 is pulled down towards circuit common, such that the FET is rendered non-conductive.
A turn-off circuit 50 is coupled to the junction of the resistors R26, R28 of the switching circuit 20 and is operable to render the FET Q22 non-conductive when either the magnitude of the voltage across the capacitor C12 reaches a predetermined peak supply voltage level or the magnitude of the AC line voltage exceeds a predetermined line voltage level. Specifically, the junction of the overcurrent protection circuit 40 and the diode D16 is coupled to the base of an NPN transistor Q52 through a resistor R62 and a zener diode Z64. When the magnitude of the voltage across the capacitor C12 and the diode D16 is such that the voltage across the zener diode Z64 is greater than the breakover voltage of the zener diode, the zener diode begins to conduct current into the base of the transistor Q52. Accordingly, the transistor Q52 is rendered conductive and the gate of the FET Q22 is pulled down towards circuit common, thus, rendering the FET non-conductive. The base of the transistor Q52 is also coupled to a line voltage detect circuit 70, which comprises a voltage divider having two resistor R72, R74. When the magnitude of the AC line voltage exceeds the predetermine line voltage level, the transistor Q52 is rendered conductive, the drain-source impedance of the FET Q22 increases, and the magnitude of the current through the capacitor C12 is limited.
After the transistor Q52 is rendered conductive during a half-cycle of the AC power source, a latch circuit 80 prevents the FET Q22 from being rendered conductive until near the end of the present half-cycle. The latch circuit 80 comprises a PNP transistor Q82 having a base coupled to the collector of the transistor Q52 through a resistor R84. When the transistor Q52 becomes conductive, the base of the transistor Q82 is pulled down towards circuit common and the transistor Q82 is rendered conductive. Accordingly, the transistor Q82 conducts current through a resistor R86 and into the base of the transistor Q52 to maintain the transistor Q52 conductive until the end of the half-cycle.
Thus, the capacitor C12 is limited to charging only at the beginning of each half-cycle. There is a need for a power supply that charges at the beginning of each half-cycle, and latches off to prevent the energy storage capacitor from charging near the peak of the AC line voltage, but also allows the energy storage capacitor to charge before the end of the half-cycle when the magnitude of the AC line voltage is below a predetermined magnitude.